While most of us have our eyes on Mars at the moment, there's a special class of astronomers who have their telescopes trained on planets a little bit farther away. Actually, a lot farther away - completely outside our solar system, in fact. We've found almost 300 extrasolar planets (or exoplanets) so far, and the search continues. Here are some surprising facts about planets that are way, way out there.

How many extrasolar planets have we found so far? 287 (as of April 1, 2008).

When was the first one found? Between 1988 and the early 1990s, several astronomers claimed to have found extraslar planets. However, the first confirmed planet was found in 1992, and the first orbiting a normal (non-pulsar) star was found in 1995.

Which one is closest to Earth? Epsilon Eridani b is a gas giant a little smaller than Jupiter that orbits a star 10.4 light years from Earth.

Which one is most likely to support life? Gliese 581 c is the smallest exoplanet found so far, and it orbits within the "habitable zone" of its star.

What is the largest planet? GQ Lup b has a mass of more than 21 Jupiters, or 70 percent of our sun's mass. In fact, there is some debate whether it is a planet or a brown dwarf star in a companion orbit to GQ Lup itself.

How do we detect extrasolar planets? It's almost impossible to find them by looking through a telescope - not because they are small, but because the contrast between the brightness of the star a planet orbits and the planet itself is too great for us to pick out the planet. We can measure the gravitational wobble induced in the star by the planet by looking at the shift in wavelength of light coming from the star (the Doppler effect) or we can examine the "gravitational lensing" effect produced when light from a background star passes through the distant solar system. In rare cases, a planet transits in front of its star (in relation to our point of view), allowing us to notice the dimming of the star.

What can we learn from these methods? By combining the data and doing some serious physics calculations, astronomers can figure out the mass and density of the planet and the characteristics of its orbit. We can even learn something about the composition of the planet and its atmosphere - the Hubble Telescope was able to detect methane in the atmosphere of a gas giant earlier this year.
Have we found any Earth-like planets? No. The majority of extrasolar planets found so far have been gas giants. We have found a few "terrestrial" planets, denser than gas giants, but they have all had five or more times the mass of Earth. Astronomers call them Super-Earths. Image by: NASA.